Continuous polyamide extraction process

ABSTRACT

A process for continuous extraction of polyamide particles in an essentially vertical extraction column using an aqueous extractant comprises using an extraction column that is divided into two zones and performing an extraction with a recirculating 15-40% strength by weight aqueous ε-caprolactam solution in the first zone and then with countercurrent water at a from 5 to 40° C. lower temperature in the second zone. The process provides for economical extraction of polyamides with a low level of monomers and oligomers.

FIELD OF THE INVENTION

The present invention relates to a process for continuous countercurrentextraction of polyamide in a vertical two-part extraction column bytreating polyamide particles with recirculating aqueous ε-caprolactamsolution.

PRIOR ART

Nylon 6 (polycaprolactam) is produced by polymerization ofε-caprolactam. The polycaprolactam obtained has a caprolactam monomerand oligomer content of, for example, from 8 to 11% by, weight. Left inthe polycaprolactam product, these low molecular weight constituentscause undesirable effects in further processing of the polymer productand must therefore be removed. Industrially, this is accomplished bycontinuous or batchwise extraction with hot water (DE-A-25 01 348,DE-A-27 32 328) and by distillative removal under reduced pressure (U.S.Pat. No. 4,376,680) or in superheated steam (EP 0 284 968 B1). Theseprocesses are all carried out with an eye to recovering and reusing theextractables for reasons of environmental protection and economy. Fornylon 6, these processes leave residual extractables(methanol-extractables) consisting essentially of caprolactam oligomerswhich are sparingly soluble in water or involatile, especially dimersand cyclic oligomers.

Various apparatus has been proposed for extracting low molecularconstituents from polyamides. GB 12 97 606 discloses an extractioncolumn that is divided into at least two zones, the extractant beingrecirculated within each zone in countercurrent to the flow of liquid byremoval at the upper end of the zone and reintroduction at the lower endof the zone. Similar apparatus is described, for example, in CZ 253 019,FR 15 18 775, DD 206 999 and DE-A-17 70 097.

It is known that caprolactam monomer acts as a solublizer forcaprolactam oligomer in the extraction of nylon 6. This is why, forexample according to JP-A-47 026438, the nylon 6 chips are pretreatedwith a solution of from 15 to 90%, preferably of from 40 to 70%, ofc-caprolactam to remove the water-solubles. In DD 289 471, the chips areinitially treated in countercurrent at above 60° C. with from 1 to 40%of caprolactam in the wash water (the percentages are each by weight).DE-A-43 24 616 discloses a process for extracting nylon 6 to obtainproducts having a very low residual level of dimeric ε-caprolactam.Here, a first stage, which features an extraction with from 41 to 80%caprolactam solution at from 80 to 120° C., is followed by one or morepostextraction stages, either with water at high temperature or underreduced pressure. In JP-A-48 002 233 polycaprolactam is purified byadmixing the molten polymer with from 5 to 30% strength caprolactamsolution and then purifying the resulting dispersion at from 80 to 120°C. in an extraction column. In JP-A-53 071 196, polyamide is initiallyextracted with a hot aqueous medium and then purified at from 10 to 50°C. below the melting point of the polyamide in an inert gas stream, thehot aqueous medium comprising, for example, water at from 80 to 130° C.with an ε-caprolactam content of less than 50% by weight. JP A-45 025519 discloses a multistage extraction process wherein the polyamidechips are extracted with from 5 to 50% strength aqueous caprolactamsolution at from 70 to 120° C. in the first stage and with from 0.1 to5% strength aqueous caprolactam solution at from 70 to 120° C. in thesecond stage. JP-A-51 149 397 describes an extraction with an aqueous60% strength by weight ε-caprolactam solution at from 80 to 120° C. forfrom 3 to 8 hours in the first stage and an extraction withcaprolactam-free water, which is preferably O₂-free or comprises smallamounts of a reducing agent, in the last stage. These processes too arepreferably carried out with recovery and reuse of the extractables forreasons of environmental protection and economy. Accordingly, JP-A-60166 324 discloses a continuous nylon 6 extractor wherein the chips areextracted with water in countercurrent by recirculating the bulk of theextraction liquid with addition of ε-caprolactam. The extractant ispumped off through an aspirator, admixed with caprolactam and returnedinto the apparatus via a distributor located at the same level as theaspirator. The residual extractables content is 1%.

In DE-A-195 05 150 the caprolactam oligomer is removed from polyamidechips by treatment with pure caprolactam as extractant at from 60 to150° C. However, this method has the disadvantage that adherentcaprolactam may lead to stickiness of the chips in subsequentoperations. Moreover, at these temperatures, the chips would alsodissolve in caprolactam to some extent.

Using water or water vapor as extractant for the polyamide chips it isvery difficult to achieve the present-day requirement of residualextractables contents <0.5%. The extract obtained will typically be asolution having an extractables content of from 5 to 15%, similar towhat is obtained using caprolactam-comprising extractants. The extractmay additionally include inorganics such as titanium dioxide, silicondioxide and manganese oxide, typically added to the polyamide forstabilization or delustering. Existing processes have in common thateither the residual extractables content of the chips is too high orthat the aqueous extract has to be highly concentrated in order that thecaprolactam monomer and caprolactam oligomer may be recycled into thepolymerization. Oligomer and inorganics may separate out during theconcentrating, which also has appreciable energy requirements.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process forpurifying polyamide to a very low residual level of monomers andoligomers without generating large volumes of extractant having a lowextractables content.

We have found that, surprisingly, this object is achieved according tothe invention by a process for continuous extraction of polyamideparticles, especially polyamide chips or flakes, in an essentiallyvertical extraction column using an aqueous extractant, which comprisesusing an extraction column that is divided into two zones and performingan extraction in the first zone with a recirculating 15-40% strength byweight aqueous ε-caprolactam solution and then in the second zone withwater in countercurrent.

The process has the advantage that the level of caprolactam oligomer inthe polyamide chips is reduced in a simple manner to obtain an extractwhich requires distinctly less workup before being feedable into thepolymerization reactor. In addition, the required low residualextractables content of <0.5%, especially <0.1% of dimer, is achieved inan economical and simple manner in a single extraction apparatus. Theextraction, moreover, provides the desired low level of oligomerappreciably faster than is the case in existing processes.

DETAILED DESCRIPTION

Suitable polyamides are polycaprolactam or copolyamides of caprolactamand further polyamide-forming starting materials, thecaprolactam-derived portion being preferably not less than 20% byweight, especially not less than 25% by weight. Preferredpolyamide-forming starting materials are diamines and dicarboxylic acidssuitable for forming polyamides. Suitable dicarboxylic acids are, forexample, alkanedicarboxylic acid having from 6 to 12 carbon atoms,especially from 6 to 10 carbon atoms, and also terephthalic acid andisophthalic acid. Suitable diamines are, for example, alkanediamineshaving from 4 to 12, especially from 6 to 8, carbon atoms, alsom-xylylenediamine, bis(4-aminophenyl)methane,2,2-bis(4-aminophenyl)propane or bis(4-aminocyclohexyl)methane.Dicarboxylic acids and diamines can each be used in any desiredcombinations, but advantageously in an equivalent ratio. Of particularindustrial significance are polycaprolactam and polyamides based oncaprolactam, hexamethylenediamine and also adipic acid, isophthalic acidand/or terephthalic acid.

Polyamide chips typically comprise from 2 to 15% by weight ofcaprolactam monomer and caprolactam oligomer, especially from 8 to 12%by weight of caprolactam monomer and caprolactam oligomer. Polyamidechips generally have a size within the range from 1.5×1.5 mm to 4×4 mm,for example have a cylindrical shape measuring about 3×2 mm.

The polyamides used may additionally include customary additives such asdelusterants, e.g. titanium dioxide, nucleating agents, e.g., magnesiumsilicate, stabilizers, e.g., copper(I) halides and alkali metal halides,antioxidants and reinforcing agents in customary amounts. The additivesare typically added before, during or after the polymerization andbefore the pelletizing step.

The polyamide chips obtained after the polymerization and subsequentpelletization are fed to the two-part, preferably tubular, extractioncolumn via a transportation water circuit, for example. The chips can beseparated from the transporting water by a separating means, forexample, and are then, customarily at a temperature of from 20 to 90°C., introduced at the top of the extraction column, i.e., into theextractor head. The chips pass downwardly through the extraction columnunder gravity and are discharged at the base of the extraction column.Water is continuously fed in at the base of the extraction column as anextractant which passes upwardly through the extraction column incountercurrent to the chips.

The extractant absorbs caprolactam monomer and oligomer in the bottomregion of the extraction column, the second zone. The extractant isrecirculated in the top part of the extraction column (extractor head).This extractor head, the first zone, accounts for 5 to 50%, preferably15 to 30%, of the total volume of the extractor. The extractant ispreferably withdrawn at the top of the first zone and reintroduced intothe extraction column in the bottom region of the first zone by adistributing means at a uniform rate. However, it is also possible toproceed conversely, i.e., the extractant is withdrawn in the bottomregion and reintroduced in the top region. The amount of extractantrecirculating within the head is chosen so that, on the one hand, atemperature and concentration equilibration is ensured within this zoneand an intensive mass transfer takes place at the phase interface of thepolyamide particles. On the other hand, the flow rate of the aqueoussolution should not exceed the swirling point of the particles.Accordingly, the velocity is generally set within the range from about 2to 20 m/h, preferably within the range from 3 to 15 m/h. In addition,the extractor head has a larger diameter than the second zone toadditionally counteract any swirling up of the particles. The ratio ofthe cross-sectional area in the first zone to that in the second zone iswithin the range from about 1:1 to 3:1, especially within the range from1:1 to 2:1. The ratio of the length of the first zone to that of thesecond zone is generally within the range from 0.05:1 to 1:1, preferablywithin the range from 0.1:1 to 0.3:1. The temperature in the extractorhead is within the range from 100 to 140° C., preferably within therange from 115 to 130° C., and is set by a heat exchanger disposedwithin the head circuit of the extractant, outside the extractioncolumn. Liquid caprolactam at from 80 to 100° C. is added to the headcircuit to set a caprolactam concentration of from 15 to 40%, preferablyof from 20 to 40%, especially of from 20 to 30%, within the extractorhead. This provides for faster and, owing to the better equilibriumposition, more thorough removal of caprolactam oligomer, especiallycaprolactam dimer, from the polyamide. The extractant is continuouslyremoved from the head circuit at the rate of the water feed at the baseof the extractor and the caprolactam feed into the first zone.

The transition from the first zone to the second zone of the extractioncolumn is preferably equipped with a flow barrier which, for example, bynarrowing the flow cross-section, prevents any sinking of the aqueoussolution, which has a higher specific gravity, from the extractor headinto the second zone underneath. For example, a honeycomb-shapedconstriction can be used to raise the superficial velocity of theascending liquid phase. The narrowing of the flow cross-sectionunderneath the extractor head additionally provides a very effectivemeans for separating the region of the first zone which is characterizedby pronounced backmixing from the second zone, in which a countercurrentconcentration profile with little if any backmixing is desired. Sinkingof the heavier extractant from the extractor head is further preventedby reducing the temperature in the second zone by from 5 to 40° C.,preferably by from 10 to 20° C., as compared to the temperature in thefirst zone. In addition, the extractant flow velocity in the second zoneis made relatively high by constructing this second, for exampletubular, zone with a very small diameter. The superficial flow velocityis customarily within the range from 0.2 to 6.0 m/h, preferably withinthe range from 1 to 3 m/h. The tube cross-section can be comparativelysmall owing to the comparatively short total residence time of from 5 to20 hours, especially of from 8 to 15 hours, required for adequateextraction. Furthermore, the resulting smaller extractor volume resultsin a relatively low, economical equipment height.

The extraction in the first zone and in the second zone is generallycarried out at a temperature within the range from 80 to 140° C. Thetemperature in the second zone is preferably lowered by from 5 to 40°C., as mentioned. However, the temperature in the second zone can alsobe higher than that in the first zone, especially if the aforementionednarrowing of the flow cross-section and a high flow velocity for theextractant in the second zone are provided.

The addition of caprolactam during the extraction serves to stabilizethe wash water obtained, so that oligomer concentrations of up to 6% arepossible in the removed extractant without troublesome precipitationsoccurring in the subsequent process. The ratio of extractant topolyamide is within the range from 0.5:1 to 2:1 in the process of theinvention. Compared with the prior extraction art without addition ofcaprolactam, accordingly, the process of the present invention requiresa smaller water polyamide ratio owing to the better equilibrium positionand the faster extraction. This reduces the amount of water to beevaporated when recovering the wash water, which improves the economicsof the overall process.

The polyamide is preferably discharged from the reactor and continuouslymetered into a transportation water circuit by a screw, especially adeep-drawn single screw. The discharged amount of polyamide and hencethe polyamide level in the extraction column can be controlled via thespeed of the screw. The discharge screw provides for a very uniform andattritionless discharge of the polyamide and prevents bridging ofparticles. Since, moreover, this form of discharge is leakage-free, thecountercurrent concentration profile in the extractor is not disturbed.The addition of small quantities of water in the transportation watercircuit, which enter the extractor through the screw, serves to create,in the screw, a flow of liquid which is countercurrent to the exitingpolyamide and at the same time ensures an upward flow of liquid phase inthe base region of the extractor, preventing any backmixing.

The polyamide treated according to the invention has a residualextractables content of less than 0.5% by weight, especially less than0.3% by weight, and a particularly low caprolactam dimer content of lessthan 0.1% by weight, especially less than 0.01% 5 by weight.

THE DRAWING

The FIGURE schematically depicts by way of example an extraction columnfor the process of the present invention. Said extraction column 1comprises a first (upper) zone 2 and a second (lower) tubular zone 3.The ratio of the length of said first zone 2 to that of said second zone3 is generally within the range from 0.05:1 to 1:1, preferably withinthe range from 0.1:1 to 0.3:1. The polyamide chips 4 are introduced intosaid first zone 2 from the top, pass through it downwardly and thenthrough said tubular second zone 3 and are then discharged via thedischarge screw 5 into the transportation water circuit 6. Water 7 isfed upwardly into the extraction column 1 through the discharge screw 5and via an annular nozzle at the base of the extractor. On passingthrough said extraction column 1, the water initially picks upcaprolactam in said second zone 3 and then mixes in the bottom part ofsaid first zone 2 with the extractant which is circulated therein. Thisis removed in the top part of said first zone 2 by a pump 8, filtered inthe filter 9, passed through a heat exchanger 10, which maintains thetemperature within the desired range and reintroduced through an annularnozzle or perforated plate 11 in the bottom region of said first zone 2.Some of the extractant is removed at 12 and sufficient fresh caprolactamis supplied via 13 that the caprolactam concentration in the extractantis maintained within the desired range. The first and the second zonesare heated via jacket heating tubes 14 and 15, respectively. Between thefirst and the second zone there is located a narrowing of the flowcross-section 16, which, together with the higher temperature in saidfirst zone 2 compared with said second zone 3, prevents any sinking ofthe heavier caprolactam solution.

EXAMPLES

The Examples which follow illustrate the process of the presentinvention. The unextracted nylon 6 chips used are from 12.5 to 14.5 mgin weight on average and have a cylindrical shape measuring about 3×2mm. They have a caprolactam monomer content of 9.0% and a dimer contentof 0.63%.

Inventive Example 1

An extraction column 1 as per the Figure has a first zone 2 4500 mm inlength and 147 mm in diameter and a second zone 3 23000 mm in length and113 mm in diameter. 20 kg/h of unextracted nylon 6 chips 4 areintroduced continuously into said first zone 2. 20 kg/h of fresh water 7at 104° C. are continuously introduced into the base of extractor 1. Insaid first zone 2 0.2 m³/h of wash water are removed using arecirculating pump 8 and, after passage through a filter means 9 and aheat exchanger 10, reintroduced into said extractor 1 via an annularnozzle or perforated plate 11, situated 4400 mm below the water surface.The temperature in said first zone 2 is set at 121° C. via said heatexchanger 10. 3.3 kg/h of liquid caprolactam 13 are metered into is theextractant circuit upstream of the heat exchanger to maintain acaprolactam concentration of about 20% in said first zone 2. Underneaththe extractor head there is situated a flow barrier 16 in the form of ahoneycomb-shaped constriction having a free diameter of 40 mm. Thisnarrowing of the flow cross-section raises the superficial flow velocityof the ascending water phase to 15.9 m/h in this region. The nylon 6chips discharged from said extractor 1 through extraction screw 5 have aresidual extractables content of 0.3% and a dimer content of 0.02%.

Comparative Example 1

Nylon 6 chips are treated in the same way as in Inventive Example 1,except that no caprolactam is added in the extractor head, affordingunder otherwise identical conditions a nylon 6 chip product having aresidual extractables content of 1.1% and a dimer content of 0.12%.

Inventive Example 2

Inventive Example 1 is repeated, except that 16 kg/h, instead of 20kg/h, of fresh water are fed in at the base of the extractor at 104° C.and 2.3 kg/h instead of 3.3 kg/h of liquid caprolactam are metered intothe header circuit. This corresponds to a water/chips ratio of 0.8. Thenylon 6 chips obtained have a residual extractables content of 0.4% anda dimer content of 0.05%.

Comparative Example 2

Inventive Example 2 is repeated, except that the extraction is carriedout with water without addition of caprolactam in the extractor head.The discharged nylon 6 chips have a residual extractables content of1.4% and a dimer content of 0.15%.

Inventive Example 3

Unextracted nylon 6 chips having an average chip weight of 6.5 mg and acylindrical shape are used under the same conditions as in InventiveExample 1. The discharged nylon 6 chips have a residual extractablescontent of 0.15% and a dimer content of less than 0.008%.

If desired, the polyamide can be further purified by known processes,for example in a simultaneous extraction and tempering as described inEP 0 284 968.

We claim:
 1. A process for continuous extraction of polyamide particlesin an essentially vertical extraction column using an aqueousextractant, which comprises using an extraction column that is dividedinto two zones and performing an extraction with a recirculating 15-40%strength by weight aqueous ε-caprolactam solution in the first zone andthen with countercurrent water in the second zone, wherein the firstzone of the extraction column has a larger diameter than the second zoneand wherein the transition from the first zone to the second zone isequipped with a flow barrier.
 2. A process as claimed in claim 1,wherein the extractant is recirculated by removing it in the top regionof the first zone and feeding it into the bottom region of the firstzone.
 3. A process as claimed in claim 1, wherein the recirculatedextractant is returned into the extraction column via an annular nozzleor perforated plate.
 4. A process as claimed in claim 1, wherein asufficient amount of the recirculating extractant is removed outside theextraction column and replaced with fresh caprolactam to establish acaprolactam content of from 15 to 40% by weight in the first zonetogether with the water introduced into the second zone.
 5. A process asclaimed in claim 1, wherein the ratio of the cross-sectional area of thefirst zone of the extraction column to that of the second zone is withinthe range of up to 3:1.
 6. A process as claimed in claim 1, wherein theratio of the length of the first zone to that of the second zone iswithin the range from 0.05:1 to 1:1.
 7. A process as claimed in claim 1,wherein the extraction is carried out at from 100 to 140° C. in thefirst zone and at a from 5 to 40° C. lower temperature in the secondzone.
 8. A process as claimed in claim 1, wherein the extraction time iswithin the range from 5 to 20 hours.
 9. A process as claimed in claim 1,wherein the extractant is added to the polyamide in a weight ratiowithin the range from 0.5:1 to 2:1.
 10. A process as claimed in claim 1,wherein the extracted polyamide is discharged from the extractor using asingle screw.
 11. A process as claimed in claim 8, wherein theextraction time is within the range from 8 to 15 hours.